The present invention relates to a polyolefin microporous film having a multiplicity of fine through-pores formed by a stretching method, a battery separator employing an improved porous polypropylene film, and a method of producing them.
Microporous film of the type which is composed of film made of polymeric material and a multiplicity of fine through-holes formed therein are used for various applications in the fields of, for example, filter membrane or separator membrane for use in air purification, water treatment or the like, a separator for use in a battery or in electrolysis, gas exchange membrane or separator membrane for use in artificial lungs or in plasma separation, and filter membrane or separator membrane for use in sterilization and refinement of various kinds of enzyme for production of hard or soft drinks such as beer or sake on draft or fresh juice.
A known method of producing a microporous film having a multiplicity of fine through-pores comprises the steps of dispersing, for example, an easily soluble substance in a polymeric material, forming the polymeric material mixed with this substance into film, and eliminating the easily soluble substance by dissolving it with a solvent, thereby forming a multiplicity of fine pores in the film.
In recent years, another method of producing a porous body has been generally accepted. This method utilizes a method which comprises the steps of forming thermoplastic crystalline polymer material into a film, then subjecting the film to heat treatment, and subsequently forming pores in the film by a stretching step.
As is known, various kinds of polymeric material can be employed as a thermoplastic crystalline polymer material suitable for the above-described purposes. Among others, polyolefin, particularly polypropylene, is inexpensive and excels in strength and chemical resistance and is therefore regarded as an excellent polymeric material suitable for use in producing a microporous film.
A method of producing a microporous film by stretching a crystalline polyolefin film and forming pores in the film is disclosed in, for example, U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,801,404; 3,801,692; 3,843,761 and 4,138,459. In these method, a film having pores connected to each other therein and having an average pore size of 1,000-2,000.ANG. can be obtained. Similar methods of producing a microporous film are disclosed in Japanese Patent Publication Nos. 46-40119, 50-2176 and 55-32531. The gist of these prior arts resides in a method which comprises the steps of subjecting an unstretched film to heat treatment, stretching the film at a temperature close to room temperature or a temperature in the range of between a temperature not lower than the second order transition temperature of the resin (for example, a temperature not lower than -40.degree. C. in the case of polypropylene) and a temperature not higher than the temperature at which the partial melting of the crystalline phase begins to occur. Thereby generating pores and forming a porous body, then stretching at a temperature in the range of between a temperature higher than the temperature at which the partial melting of the phase begins to occur and a temperature not higher than the crystalline melting point, and again conducting heat treatment to thermally fix the pores thus formed.
However, in the conventionally known microporous film produced by the stretching process, the pores are defined by relatively thick portions of nonuniform sizes and configurations which form meshes of various sizes and meandering shapes and fine fibrils which run in these meshes in the direction approximately parallel to the direction in which the film is stretched. Accordingly, the configurations and sizes of the pores are nonuniform and the porosity is low. In addition, since the meandering portions form continuous closed circuits of various sizes, the longitudinal lengths of the pores which extend through the film from one to the other surface are excessively long. Accordingly, if the microporous film obtained by the prior art is used for plasma separation, the plasma filtration rate is low and the selective separating capability is inferior.
A nonwoven fabric of synthetic resin or a nonwoven fabric of glass fiber is generally employed as a battery separator. In recent years, electronic devices have been made portable or even smaller in size, weight or thickness, and various kinds of card-like electronic devices have been being developed. In such a situation, in the field of batteries for use as the electrical power sources of such electronic devices, a demand has arisen for an increase in the energy density and a reduction in the thickness of the battery. One method for meeting the demand is to improve a separator which is one element of a battery. More specifically, a method for using a porous film as a battery separator is proposed in order to answer the demand for an improvement in battery performance and a reduction in the thickness of the battery based on an increase in the amount of active material enclosed or a reduction in internal resistance. For example, Japanese Patent Laid-Open Nos. 62-222562 and 63-126159 disclose a method of preparing a separator by impregnating a porous polypropylene film with a liquid electrolyte or a semi-solid electrolyte.
The porosity of porous polypropylene film which has heretofore been used is at most 45% or thereabouts. As described above, typical methods of producing conventional porous polypropylene film are disclosed in Japanese Patent Publication Nos. 46-40119, 50-2176 and 55-32531. The porous polypropylene films obtained in these methods, however, present a number of problems. For example, since the porosity is low, electrolyte cannot be sufficiently retained and an area which allows ionic conduction is limited. In addition, since the overall path lengths of the pores from one surface of film to opposite side of film are excessively long and the substantial distance between the electrodes is large, the internal resistance increases. If a separator made from such a film is used in a battery, the performance of the battery will be low.
If one intends to increase the aforesaid porosity with the prior art method of producing a porous polypropylene film, porosity will not increase beyond some limiting value being less than about 55%, and nonuniform pores will be formed in the film. If such a film is used as a separator, fine powder in an active material of either electrode may pass through the separator and contact the other electrode, thus resulting in a deterioration in the battery performance.